If alien life does live in the warm, hydrothermal oceans of such icy moons as Europa or Enceladus, we're now a step closer to being able to identify it with accuracy. Amino acids are the building blocks of life and so could be a key signature of the existence of alien life on other planets. However, the protein-building chemicals can also form through natural geological processes — such as, for example, those that formed the solar system.

Researchers from the US have now ruled this out as a possible source for any amino acids that could be found in the icy moons of our solar system. They found that amino acids almost entirely decompose quickly compared to the age of the solar system. This means that any amino acids found on Europa of Enceladus today must be the product of either other, recent, geological processes or alien lifeforms.

'Amino acids are the building blocks of protein, and the chemistry of life as we know it,' geoscientist Ngoc Truong of New York's Cornell University told New Scientist. Because of this, amino acids are key signatures in the hunt for extra-terrestrial life, such as on the icy moons of Jupiter's Europa and Saturn’s Enceladus. However, they can also be formed through a variety of geological processes including those that first made the solar system. 'I wanted to see if amino acids can be found coming from the ocean of Europa, could they be the relics of primordial synthesis processes?' said Mr. Truong.

Mr. Truong teamed up with colleagues from Cornell University, Arizona State University and the Southwest Research Institute to measure how long 14 amino acids that can be used to create proteins can last in water before decomposing. They found that in the warm, hydrothermal oceans of icy moons, certain amino acids would decay relatively quickly in comparison with the age of the solar system. In particular, aspartic acid and threonine would only remain in such oceans in concentrations of above one nanomolar if these amino acids were synthesised in the last billion years.

Given this, Mr. Truong and colleagues conclude, any of these amino acids detected on moons like Europa and Enceladus would have to have been produced relatively 'recently', at least when viewed in the context of the solar system's overall history. This would eliminate the possibility that these chemicals could be remnants of the solar system's formation 4.6 billion years ago. However, detecting these amino acids on Europa or Enceladus would not be conclusive proof of life on the moons. It would still need to be shown that any such chemical signatures found have not been produced instead by geological processes, such as through the interaction between water and minerals.

Enceladus is Saturn's sixth largest moon, at 313 miles (504 kilometres) wide. It is an icy satellite with hydrothermal activity - a rare combination - with vents spewing water vapour and ice particles out from a global ocean buried beneath the moon's frozen crust. A handful of worlds are thought to have liquid water oceans beneath their frozen shell, but only Enceladus sprays its ocean out into space, where a spacecraft can sample it. According to NASA observations, the plume includes organic compounds, volatile gases, carbon dioxide, carbon monoxide, salts, and silica.

Microbes on our planet either produce these compounds or use them for growth, leading some to speculate that tiny organisms live in Enceladus's hidden ocean. This means that while Enceladus may look 'inhospitable' like Saturn's other moons, it is a prime candidate in our search for alien life.

'This paper is exciting because it helps us understand which biosignatures we may wish to target for future missions to Enceladus or Europa to search for life,' said NASA scientist Morgan Cable, of the Jet Propulsion Laboratory in California, who was not involved in the present study. 'If we find them there that will be a really compelling reason to go back and dig deeper,' Cable added.

One such mission is NASA’s Europa Clipper, to be launched in 2023. The spacecraft will be equipped with two mass spectrometer instruments to enable it to look for signs of amino acids in the plumes of water that the Jovian moon periodically ejects from its oceans. 'We’ve done experiments in the laboratory to see what kind of fingerprints you expect from all kinds of amino acids,' said Free University of Berlin researcher Frank Postberg, who is helping to develop a mass spectrometer for the Europa Clipper.

'It’s a pretty clear fingerprint, even at low concentrations,' he added.

WHAT DO WE KNOW ABOUT EUROPA AND WHY IS IT SO SPECIAL?

Jupiter's icy moon Europa is slightly smaller than Earth's moon. Europa orbits Jupiter every 3.5 days and is tidally locked - just like Earth's Moon - so that the same side of Europa faces Jupiter at all times. It is thought to have an iron core, a rocky mantle and a surface ocean of salty water, like Earth. Unlike on Earth, however, this ocean is deep enough to cover the whole surface of Europa, and being far from the sun, the ocean surface is globally frozen over.

Many experts believe the hidden ocean surrounding Europa, warmed by powerful tidal forces caused by Jupiter's gravity, may have conditions favourable for life. Nasa scientists are on the verge of exploring Jupiter's ocean moon Europa for signs of alien life. Europa is our best shot of finding biological life in the solar system, researchers say. The space agency is priming two probes, including one that will land on its surface, to explore the distant moon in detail within the next decade, the agency says.